This invention relates to precipitation detectors and particularly to apparatus and method for detecting wet and icy conditions on the surface of a pathway, such as a highway or airport runway.
Apparatus which detect wet and icy surface conditions are of tremendous aid to motorists, highway departments, airport managers, and the like. Several such systems have been proposed, including those disclosed in coassigned U.S. Pat. Nos. 3,873,927, 3,882,381 and 4,135,151. Other patents in the field include U.S. Pat. Nos. 2,419,266, 3,243,793 and 3,428,890, and French Pat. No. 2,078,982.
While prior systems provide useful information on surface conditions, it would be advantageous if their effectiveness could be improved in certain respects. For example, it has been found that the system of U.S. Pat. No. 4,135,151 is so sensitive to impurities such as salt or the like on the surface of the sensor block when the apparatus is operated at a frequency of 5 KHz that moisture is detected when the surrounding pathway is dry. Prior systems, which operate at substantially higher frequencies, e.g., 200 KHz, are not this sensitive to impurities but are also not as sensitive to pure water on the surface of the sensor block. Operation at these higher frequencies can result in pure water going undetected. In addition, the response of prior systems is frequently subject to drift and variation as a function of temperature.